This invention relates to the manufacture of molded materials from finely powdered plant materials containing lignin. In particular, the invention provides a method of making a high density molded thermoset powdered plant material with characteristics and qualities similar to engineering grade thermoplastics and thermoset materials. Plant fibers of less than 500 microns in size are compressed into resilient, molded materials. Products manufactured by using the method of the invention are also described.
In the systems of the prior art, long strands, fibers, flakes or chips of wood are commonly used to manufacture low and medium density boards, felts or other materials for building and other uses. However, this conventional technology has focussed on physically bonding such pieces into agglomerations forming the boards, felts and other materials. The strength characteristics of the final products were ultimately limited by the strength of the individual fibers that had been bonded or glued together and the interfacial bonds between the fibers and the glue. Typically, wood fibers, chips, and flakes much larger than 3000 microns were used as a raw material source for these conventional manufacturing techniques.
Furthermore, prior art systems typically employed multiple stages to form the desired products. For example, intermediate felts and other shapes would be formed and would then be subjected to additional chemical or physical treatments including calendaring, pressing, dewatering or other processes.
In general, wood treatment related technologies have developed separately from efforts to utilize other naturally occurring plant materials. Whether in the field of wood processing technology or in the processing of other plant materials, those efforts have taught and advanced the use of larger raw material particles of sizes averaging well above 3000 microns.
One attempt at physically bonding somewhat smaller particles of straw is briefly described in UK patent application number GB 2 265 150 A, dated Sep. 22, 1993 by Brian Harmer (hereafter called xe2x80x9cHarmerxe2x80x9d). However, that reference teaches the use of straw fibers within a broad range of fiber sizes, all of which are much larger than the plant fibers of the present invention. Indeed, Harmer, teaches the use of a different process using much larger straw fibers of various sizes within a broad range of more than 500 microns and up to about 3000 microns. Harmer teaches that straw particles within a range of 500 microns to 2000 microns are preferred. Harmer, like many references in the area of wood fiber technology, teaches away from the use of very fine powders of less than 500 microns in diameter. Further, Harmer teaches the use of styrene to form a protective outer skin on the resulting product to inhibit water absorption.
In addition, the use of a broad range of particle sizes of up to 3000 microns in that process will result in a final product with a highly textured surface having discreet particles which are clearly visible to the naked eye. In part, the use of larger straw particles was taught by Harmer as a means of avoiding difficulties associated with that process, including the use of a two stage phenolic resin and hexamine as a cross linking agent. The phenolic glue system, once polymerized, produces a physical bond between the fibers and the glue. To reinforce this physical bond, Harmer uses hexamine as a crosslinking agent to enhance the physical bonding characteristics. Also, Harmer does not teach how to avoid problems associated with the application of conventional mixing techniques to satisfactorily combine a powdered two stage phenolic resin including hexamine with very finely powdered straw fibers of sizes below 500 microns. Harmer also does not teach how to avoid premature reactions of liquid additives or other powdered additives which may be included in a plant fiber formulation.
In the present invention, very finely powdered lignocellulosic plant fibers of below 500 microns are used. Typically, such fibers will have a maximum length of 500 microns, with particle diameters ranging between about 20 to 50 microns. It is understood that such particles are irregularly shaped, within a broad range of sizes of up to 500 microns in effective size. In many applications, plant fibers of less than 250 microns will be preferred. It will be understood by those skilled in the art that the size of such particles will typically fall within a range of particle sizes characterized by screening or other suitable grading techniques. In some instances, the size of such particles is referred to as an effective diameter, or effective size however, the actual size of a given irregularly shaped particle will not necessarily correspond to the effective size of the particle. Rather, the effective size will relate to the tendency of the particle to pass through a sieve or other screening or grading device.
Plant fiber particles containing lignin are desired to enhance the binding characteristics of the thermoset binding agents described further below.
Finely powdered wood fibers derived from hardwoods and softwoods may be used provided they have not been pretreated to remove significant amounts of lignin and related naturally occurring components of wood. Other suitable lignocellulosic materials include finely powdered flax, hemp, grasses, jute, and various agricultural products and waste plant materials containing lignin.
The finely powdered plant fibers are preferred to have a moisture content of less than about 50 per cent by weight and more preferably, between about 5 per cent to about 20 per cent by weight. For example, in processes utilizing polymeric diphenyl methane di-isocyanate, substantial concentrations of moisture in the plant fibers will enhance bonding within the plant fiber mixture.
According to the method of the present invention, the finely powdered plant fibers are mixed with a thermoset binding agent, and preferably, a release agent. The plant fiber and additive mixture is introduced to a heated mold operating between 40 degrees C. and 300 degrees C. In certain systems, lower reaction temperatures of about 40 degrees C. will be effective at relatively higher pressures. For example, binding agents such as polyester resin in plant fiber may be mixed with organic peroxide in plant fiber at about 40 degrees C. In heat sensitive binding agent systems, operating temperatures of up to 300 degrees C. may be applied for relatively short pressing cycles. In such cases, some degree of surface charring or other imperfections may arise. Such imperfections may be removed by subsequent operations, or may remain if they will not detrimentally affect the product""s expected performance. Preferred operating temperatures range between 100 degrees C. and 220 degrees C., and more preferably between 160 degrees C. and 220 degrees C.
The contents of the mold are heated and compressed under pressures of at least 500 psi, with preferred operating pressures greater than 1000 psi and higher.
The resulting products have average densities of at least 60 pounds per cubic foot. Higher average product densities of more than 80 pounds per cubic foot and more than 90 pounds per cubic foot are also provided. Higher product densities will in many instances provide for enhanced physical and mechanical characteristics. Such characteristics will correspond to specific formulations and may include one or more of such properties as increased strength, impact and wear resistance, decreased water absorption, and increased dimensional stability.
In one embodiment of this invention, a high density plant material is manufactured by a method comprising the steps of:
(a) introducing into a mold a mixture comprising powdered plant fiber particles of less than 500 microns, thermoset binding agent between at least 0.1 per cent and 50 per cent by weight of the plant fiber particles;
(b) operating the mold at a temperature between 40 degrees C. to 300 degrees C.;
(c) applying a pressure of at least 500 psi to the contents of the mold;
(d) compressing the contents of the mold to an average density of at least 60 pounds per cubic foot; and
(e) releasing the contents from the mold.
Internal or external mold release agents may be used in those applications requiring a release additive. An external mold release agent may be introduced to the mold separately from the plant fiber mixture. Alternatively, mold release additives may be added to the plant fiber mixture to be compressed within the mold. Although a mold release may be desirable in many instances, such additives may not be required in all applications.
In another embodiment of this invention, a high density plant fiber product is formed by using a method comprising the steps of:
(a) mixing one or both of (i) a first amount of powdered plant fiber of less than 500 microns and a thermoset resin and (ii) a second amount of powdered plant fiber of less than 500 microns and one or more additives;
(b) preparing a plant fiber mixture containing thermoset resin in a concentration of between 0.1 per cent and 50 percent by weight of powdered plant fiber comprising mixing one or both of the first and second amounts with other additives;
(c) introducing the mixture of plant fibers and additives into the cavity of a mold;
(d) compressing the mixture by applying a pressure of at least 500 psi to the surface of the mixture;
(e) heating the mold cavity to between 40 degrees C. to 300 degrees C.;
(f) compressing the contents of the mold to a density of at least 60 pounds per cubic foot; and
(g) removing the compressed contents from the mold.
A combination of one or more of mineral and non-mineral additives may be provided to enhance the process or the performance characteristics of the final products. By way of example, such additives may include one or more synthetic additives including, synthetic catalysts and synthetic pigments, glass microspheres, glass fibers, carbon fibers, aramid fibers, metallic particles and other compatible additives. The use of these additives may provide enhanced product strength, impact resistance, wear resistance, dimensional stability and other favourable product qualities. Concentrations of additives in plant fiber mixtures of up to 50 per cent by weight of fiber are provided. In one aspect of this invention, mineral additives, including silicate additives, silica or silica sand, in concentrations up to 50 percent by weight of plant fiber, are provided. Coupling agents may be added to improve the bonding of the inert mineral and non-mineral additives within the final product.
In another aspect of this invention, a plant fiber product is formed by molding a desired shape to an average density of at least 60 pounds per cubic foot. The product is made substantially from powdered plant fibers containing protolignin, a thermoset binding agent in a concentration of between about 0.1 per cent and 50 per cent by weight of plant fiber, and a release agent. The fibers have an effective size of less than 500 microns.
In another aspect, the invention includes a plant fiber product mixture comprising protolignin containing plant fibers of between 20 and 500 microns in size, a release agent, and a concentration of binding agent of less than 50 per cent by weight of plant fibers.